Which process is used for energy when oxygen is absent?

Which process is used for energy when oxygen is absent?

• A. Glycolysis
• B. Anaerobic respiration/fermentation
• C. Photosynthesis
• D. Cellular respiration

Answer: (B)

Without oxygen, cells switch from relying on oxidative phosphorylation to pathways that generate energy without oxygen. Glycolysis, the initial stage that breaks glucose into pyruvate, happens without oxygen and supplies a small amount of ATP, but to keep glycolysis going when O2 is absent, NAD+ must be regenerated. Fermentation does this by converting pyruvate into lactate in animals or into ethanol and CO2 in yeast, which allows glycolysis to continue and produce about two ATP per glucose. In many organisms, anaerobic respiration uses alternative final electron acceptors, but the common outcome is energy production without oxygen. So the process used for energy when oxygen is absent is anaerobic respiration/fermentation.

Without oxygen, cells switch from relying on oxidative phosphorylation to pathways that generate energy without oxygen. Glycolysis, the initial stage that breaks glucose into pyruvate, happens without oxygen and supplies a small amount of ATP, but to keep glycolysis going when O2 is absent, NAD+ must be regenerated. Fermentation does this by converting pyruvate into lactate in animals or into ethanol and CO2 in yeast, which allows glycolysis to continue and produce about two ATP per glucose. In many organisms, anaerobic respiration uses alternative final electron acceptors, but the common outcome is energy production without oxygen. So the process used for energy when oxygen is absent is anaerobic respiration/fermentation.